Evolutionary and developmental aspects of phalangeal formula variation in pig-nose and soft-shelled turtles (Carettochel
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ORIGINAL ARTICLE
Evolutionary and developmental aspects of phalangeal formula variation in pig-nose and soft-shelled turtles (Carettochelyidae and Trionychidae) Massimo Delfino & Uwe Fritz & Marcelo R. Sánchez-Villagra
Received: 7 September 2009 / Accepted: 17 November 2009 / Published online: 6 March 2010 # Gesellschaft für Biologische Systematik 2010
Abstract In order to examine the evolution of the phalangeal formula in a diverse clade of turtles, including hyperphalangy as a rare condition in this group, we studied 210 specimens representing all extant genera of Trionychidae and their sister taxon, Carettochelyidae. Both groups consist of highly aquatic species with elongated autopods that are either paddle-like (Trionychidae) or transformed to flippers (Carettochelyidae). Phalangeal formulae were obtained mostly by radiographs of alcohol-preserved or dry specimens, as well as by direct counts from skeletons. All trionychids and Carettochelys are pentadactylous, but their phalangeal formulae differ. Carettochelys exhibits the turtle-plesiomorphic state (manus and pes: 2-3-3-3-3), with no variation in adults. Trionychids exhibit intraspecific variation, ranging from 2-3-3-3-2 to 2-3-3-6-5 for the manus, and from 2-3-3-3-2 to 2-3-3-5-3 for the pes. The extant Carettochelys as well as the Middle Eocene Allaeochelys crassesculpta are characterized by an elongation of phalanges, whereas trionychids consistently have shorter phalanges. All trionychid genera exhibit some degree of hyperphalangy in digits IV and V, in both the M. Delfino : M. R. Sánchez-Villagra (*) Paläontologisches Institut und Museum, Universität Zürich, Karl-Schmid-Straße 4, 8006 Zürich, Switzerland e-mail: [email protected] M. Delfino Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy U. Fritz Museum of Zoology (Museum für Tierkunde), Senckenberg Dresden, A. B. Meyer Building, 01109 Dresden, Germany
manus and pes. Phalanges of the clawed digits I–III are very robust compared to phalanges of the non-clawed digits IV and V. The latter contribute significantly to the enlargement of the paddle by their additional phalanges. We hypothesize that this phalangeal pattern is coupled with prolongation of growth processes in the non-clawed digits. The differences in autopod morphology between carettochelyids and trionychids reflect different locomotor patterns related to different natural histories (elongated flippers for high-speed escape in the mainly herbivorous Carettochelys; broad paddles for rapid turns during hunting in the mainly carnivorous trionychids). The autopod of Pelodiscus sinensis is proposed as an experimental model to examine the developmental basis of adult autopod variation. Keywords Cryptodira . Growth . Heterochrony . Limb . Skeleton
Introduction Despite being perceived as structurally constrained by the presence of the shell (Renous et al. 2008), chelonians have successfully diversified to a variety of forms adapted to both aquatic and terrestrial habitats. Such ecological diversit
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